Preparation and Characterization of Near-Infrared Luminescent Bifunctional Core/Shell\nNanocomposites

Abstract

Bifunctional nanocomposites with superparamagnetic and NIR luminescent properties were synthesized by a\nlayer-by-layer and a modified Stöber method. Fe₃O₄ nanoparticles as the core were coated with NaYF₄:Ln³⁺\n(Ln = Nd, Er, Pr, or Ho) to form the first layer. Then, the second layer was coated with silica to improve the\nchemical stability and photostability. The X-ray diffraction patterns showed that a cubic spinel structure of\nFe₃O₄ and the coexistence of a cubic and hexagonal structure of NaYF₄ were obtained. Energy dispersive\nX-ray (EDX) spectroscopy analysis confirmed the core/shell structure of Fe₃O₄@NaYF₄:Ln³⁺. Transmission\nelectron microscopy images revealed that the bifunctional nanocomposites consisted of crystalline Fe₃O₄@NaYF₄:Ln³⁺ cores and amorphous SiO₂ shells, in a spherical shape with a narrow size distribution. Magnetic\nmeasurements showed that the obtained bifunctional nanocomposites exhibited superparamagnetic behavior.\nEmission spectra indicated that the bifunctional nanocomposites possessed a high near-infrared (NIR)\nluminescent intensity. Moreover, the hexagonal phase NaYF₄:Ln³⁺ showed NIR emission 10 to 15 times\nstronger than the cubic phase.